GENEVA — After several days of optimistic reports that negotiations with Iran were on track to produce the first agreement in a decade to freeze its nuclear program, the talks ended early Sunday without an agreement, the French foreign minister said.

The talks hit a snag on Saturday with a French objection that the proposed deal did not do enough to curb Iran’s nuclear ambitions.

Even as U.S. diplomats made a final push for an agreement late Saturday, the marathon talks laid bare the challenge of drafting a deal that would satisfy both the Iranians and a group of major powers with their own interests and agendas.

U.S. Secretary of State John Kerry huddled for hours with Iran’s foreign minister, Mohammed Javad Zarif, as the United States struggled to close gaps on issues like curbing Iran’s uranium enrichment program and a nuclear reactor, under construction, that will produce plutonium.

Most Read Stories

Signs of division first emerged when the French foreign minister, Laurent Fabius, said a draft of a potential deal was unacceptable to France and there was no certainty that this round of negotiations would lead to an agreement. “We are hoping for a deal, but for the moment there are still issues that have not been resolved,” he told France Inter radio.

His comments came amid a whirl of diplomatic activity, with Kerry and foreign ministers from Britain, France, Germany, Russia and China engaged in round-robin meetings with Zarif and the European Union’s foreign policy chief, Catherine Ashton, who is overseeing the talks. Kerry also met with the Russian foreign minister, Sergey Lavrov.

Zarif insisted that there had been progress, although he conceded that the diplomats might leave this round empty-handed. “That won’t be a disaster,” he said in an interview with the BBC, “because we have started an important process and, provided that we can continue this process and try to reach positive results, I think we’ve done extremely important work.”

Hopes that a deal was at hand surged when Kerry cut short a trip to the Middle East to fly to Geneva on Friday. But he, too, sought to temper expectations, saying after he arrived that an agreement had not yet been reached and that gaps needed to be narrowed. On Saturday, Kerry made no further comment before a two-hour meeting with Zarif.

While talks continued Saturday afternoon, it increasingly appeared that the negotiators would be unable to overcome gaps in this round, and officials said they hoped to return in coming weeks to try again.

U.S. officials said they sensed an opportunity to wrap up an interim accord that would freeze Iran’s program for perhaps six months so there would be time for both sides to reach a more lasting agreement. But they also said the United States was ready to meet again in a couple of weeks should the remaining differences prove hard to overcome.

“It’s important that Iran knows we’ll walk away if our concerns aren’t met,” a senior administration official said, “but we do have substantive outlines set well enough that it’s worth trying to narrow gaps.”

France has taken a harder line than the United States in recent years on curbing Iran’s capacity to produce nuclear fuel that could be used in weapons.

Diplomats said the French were particularly concerned about the heavy-water reactor being built near Arak, because it would produce plutonium, an alternative to uranium for fueling a weapon.

Daryl Kimball, executive director of the Arms Control Association, said the plant could be dealt with in a future phase of the talks because it would take a year for it to be completed and much more time for it to produce plutonium that could be extracted for a bomb.

But Kerry said during his visit to Israel last week that the United States was asking Iran, as part of an interim accord, to agree to a “complete freeze over where they are today,” implying that Iran’s plutonium production program would be affected in some way as well.

Under a compromise favored by some U.S. officials, Iran might agree to refrain from operating the facility for six months, while continuing to work on the installation.

Once the reactor at Arak is operational, as early as next year, it might be very hard to disable it through a military strike without risking the dispersal of nuclear material. That risk might eliminate one of the West’s options for responding to Iran and reduce its leverage in the talks.